The relationship of various carcinogens and structurally similar non-carcinogens are being examined in relationship to immune alteration. We have examined benzo(a)pyrene and benzo(e)pyrene as well and methyl and ethyl carbamate pairs. These studies have shown that B-cell but not T-cell function are impaired by the carcinogens but not the non-carcinogens. Urethane (ethyl carbamate) appears to have a preferential affect on the bone marrow. Myelotoxicity can be demonstrated far below levels which induce general toxicity or immunotoxicity.